The present invention generally relates to the field of radio frequency circuits, filters, antenna beamforming, mobile communications network stations, and satellites, and more particularly relates to analog implementations of circuits that couple transceivers to antennas.
Orthogonal Frequency Division Multiplex (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) are digital data communications techniques that originated before the 1970's as a method of transmitting data over HF Ionosphere radio paths and are enjoying a new resurgence in connection with possible future public wireless communications systems.
Early OFDM system were known variously as multi-tone modems or Kineplex, and divided a data stream for transmission into perhaps 32 low data rate streams, which were each modulated onto one of 32 orthogonally spaced subcarriers. The generation of 32 modulated carriers was carried out by applying 32 symbol-representative signal values to the inputs of a 32-point Discrete Fourier Transform circuit, likely implemented as a Fast Fourier Transform, and the 32 output values of the Fourier Transform, when transmitted sequentially in time, form the desired 32-tone signal. In these early implementations, even an FFT of size 32 was a significant digital signal processing computational burden, because of the embryo state of digital integrated circuits. Today, the proposed sizes of FFT for future OFDM systems is of the order of 1024-4096, and while modern digital signal processors have advanced significantly in speed and performance, the frequency with which FFTs of this size must be performed, such as every 63.5 microseconds in one proposal, is still a burden for a small, battery-powered device such as a cellphone. The solution is likely to be a custom chip design block that efficiently implements all or key operations of an FFT. Current thinking is that such an implementation would be exclusively in digital logic for carrying out numerical arithmetic. However, future products could benefit from other solutions to render the need to perform large Fourier Transforms frequently less burdensome in cost and power consumption. Therefore, analog and passive circuits are shown herein to offer such advantages.
It was also known in the prior art that a passive radio frequency circuit could be constructed with N input ports and N output ports, such that the RF signal into input port (k) divided between the output ports with phase shifts 0, 2πk/N, 2(2πk/N), . . . , (N−1)((2πk/N). The network therefore has the N×N port transfer function of a Fourier matrix. A passive RF network of hybrids and couplers realizing such a network is known as a Butler Matrix, and is often used in connection with antenna beamforming. The prior art Butler matrix was generally constructed with N a power of two, and the resulting structure resembles the flow diagram of a base-2 Fast Fourier Transform. RF Butler matrices operate within a limited bandwidth around a carrier frequency and do not work at baseband, e.g., down to zero frequency.